If your brakes start making a grinding or scraping noise every time you use them, check the brake blocks immediately. They will either be worn right down to the metal, or there will be small stone chippings or other materials embedded in the rubber of the brake block.
Worn blocks should be replaced immediately. If you put off replacing them you are in danger of wearing the side walls of your rims away. Some brake pads/blocks can be pricey, a new wheel or rim replacement will be even more expensive.
Debris in the brake block can normally be removed fairly easily using a pen knife or similar. If the pads have plenty of life left in them it is worth giving them a rub over with a clean wire brush in order to remove as much debris as possible.
Don’t forget to clean the braking surface of the rim as well – use a Scotchbrite pad (green pan scourer) and some warm water for this – you will be amazed at how much improvement there is in your bikes stopping power.